Allow system to disable behavior of pausing animators for bg apps

This change adds a static method which can be called to disable the
default behavior of pausing infinite animators when an app's windows
are all in the background. This could potentially be used for
global behavior of a system property to disable this behavior system
wide.

Bug: 232937493
Test: Added new cts test to AnimatorLeakTest to verify behavior
Change-Id: Idf4957e3968253228096671fde89f820311883e3